1. Field of the Invention
This invention relates to an ultrasonic oscillator and to an ultrasonic motor which drives a rotor or the like using the ultrasonic oscillator.
2. Description of the Related Art
Examples of ultrasonic motors of this type are disclosed in Japanese Patent Laid-Opens No. 61-52163 and 63-87184.
According to the disclosure of Japanese Patent Laid-Open No. 61-52163, an ultrasonic oscillator performs a beat/reflex oscillation which is a combination of beat mode and reflex mode oscillations, the latter exhibiting inverted phases on both sides of the longitudinal axis of a cantilever. A rotor is pressed against the end surface of the ultrasonic oscillator so as to be rotated with the surface.
According to the disclosure of Japanese Patent Laid-Open No. 63-87184, on the other hand, piezoelectric oscillators are provided on one end surface of a resonator to form an ultrasonic oscillator, oscillations in the thickness direction of the piezoelectric oscillators causing the ultrasonic oscillator to resonate in the longitudinal direction with half or an integral multiple of the wavelength thereof. Provided on the other end surface of the ultrasonic oscillator is one or a plurality of flexural-oscillation protrusions adapted to generate a flexural oscillation exhibiting a unilateral amplitude in response to the resonance of the ultrasonic oscillator, the flexural-oscillation protrusions being arranged around the axis of rotation of a rotor in such a manner that their free-end sections are in contact with the rotor.
The above-described conventional ultrasonic motors have, however, the following problems: in these conventional ultrasonic motors, the section where longitudinal oscillations are amplified and the section where lateral oscillations are generated are provided separately in terms of function, resulting in a complicated structure. Furthermore, since the mechanism for generating longitudinal oscillations and that for generating lateral oscillations are combined with each other in a separated form, a step section inevitably exists between them, which step section involves oscillatory energy loss, a serious structural defect in an ultrasonic motor. In the case of an ultrasonic motor using oscillation protrusions (Japanese Patent Laid-Open No. 63-87184), the following problem is, in particular, involved, in addition to the above-mentioned: that is, the oscillation protrusions are individually arranged on the resonator, so that, if a load is applied to one of the oscillation protrusions alone, only that protrusion to which the load is applied serves to rotate the rotor, the other protrusion oscillating in what is for practical purposes an unloaded state. Consequently, the oscillatory energy of this unloaded protrusion is hard to transmit to the loaded one.